High pressure conduits, such as oil and gas pipelines, have generally been constructed with conventional steel pipes. These pipelines are subjected to both internal and external pressures. Internal pressure is required to transport the fluids or gases within the pipeline. External pressure is created by the weight of soil or water on the pipeline when the pipeline is buried underground or submerged in water.
While steel pipes provide the requisite strength for withstanding the internal and external pressure, they have a high susceptibility to corrosion. A corrosive environment is fostered by contact between internal foreign media (e.g., the electrolytic liquids or gases being transported by the pipeline) and the steel, or by contact with external conductive foreign media and the steel. The external foreign media could be soil in cases where the pipe is buried underground, or sea water in cases where the pipe is submerged in an ocean, or water in cases where the pipe runs along sewer systems or is exposed to rain. Corrosion decreases the pipe's strength and may cause the pipe to leak or burst under pressure.
To overcome this disadvantage, steel reinforced composite pipes have been developed. These pipes have a wall of steel coated with a polymeric material, or of steel embedded in a fiber reinforced composite, such as a fiberglass-resin system. The coating or resin system protects the steel from corrosion by shielding it from any contact with the foreign media. One example of steel reinforced composite pipe is disclosed in the Cocks U.S. Pat. No. 4,351,364, the subject matter of which is hereby incorporated by reference. The pipe disclosed has a structural wall section sandwiched between inner and outer linings. The linings are resin-rich layers reinforced with glass or other fibers. The structural wall section is made of three or more structural steel reinforcing layers coated with structural epoxy resin. The individual layers of the pipe are successively built up, one upon the other, on a mandrel or pipe winding machine. Each lining layer is formed by helically winding resin wetted fiber rovings. Each steel layer is formed by helically winding a steel strip coated with resin.
Since most of these pipelines are buried or submerged, visual inspections, of the outer surface at least, is a difficult and expensive proposition. The soil burying the pipe would have to be excavated in cases where the pipe is buried underground. In cases where the pipe is submerged in water, divers may have to do the visual inspection. Visual inspection within a pipeline poses obvious difficulties.
Even if visual inspections were possible, the onset of steel corrosion may not be visually detectible, since the steel layers are coated or covered with a fiber-resin system. In many cases, the corrosion becomes visually detectible only when it has progressed far enough to affect the protective layers. It is possible that the pipes will leak or burst prior to the corrosion ever becoming visually detectible. It is imperative, therefore, that any contact of the steel with a corrosion fostering foreign medium be detected so that timely action is taken to prevent the corrosion onset. A system capable of detecting such contact would reduce pipeline inspection costs and reduce pipeline failures. To be successful, such a system must be capable of functioning without interfering with the pipeline operation. In other words, the pipeline operation should not have to be stopped every time a pipe section needs to be inspected. Otherwise, the costs involved in stopping the pipeline operation may make such a detection system economically infeasible.